Although an essential arm of innate immunity, activated neutrophils contribute greatly to the tissue damage associated with lung injury and inflammation. As neutrophils advance from the vasculature, through the interstitium, and across the epithelial barrier, the go through progressive stages of activation. Thus, it is reasonable that specific interactions with resident cells and molecules regulate and constrain the activation of neutrophils as these granulocytes move through different tissue compartments. Data in support of this concept demonstrate that the transepithelial migration and activation of neutrophils in injured lung are blocked in mice lacking matrilysin, an epithelial-produced matrix metalloproteinase (MMP). These studies also showed that the CXC chemokine KC, a potent, acute-phase neutrophil chemoattractant, is bound to the glycosaminoglycan (GAG) chains of syndecan-1, a transmembrane heparan sulfate proteoglycan on the basal surface of lung epithelial cells. Following injury, matrilysin sheds these complexes from the epithelial cell surface. The central hypothesis of this project is that interaction with cell-bound KC/syndecan-1 complexes constrains neutrophil movement and, importantly, activation, thereby preventing a damaging oxidative burst at the epithelial cell surface. In contrast, interacting with soluble KC/syndecan-1 complexes promotes neutrophil activation, ideally at a safer distance from the mucosal layer. For Aim 1, the response of matrilysin-null and syndecan-1-null to lung injury and infection will be assessed in detail, and high resolution imaging and markers of neutrophil-mediated oxidation will be used localized where and what stage neutrophil activation is halted in these mouse stains. For Aim 2, transgenic mice, transfected cells, and purified reagents will be used to test the idea that neutrophils respond differently to immobilized or cell-fixed versus soluble KC/syndecan-1 complexes. For Aim 3, the interaction between KC and syndecan-1 GAG chains will be mapped to design reagents to block this interaction and, in turn, neutrophil activation. Relevance to Public Health Concerns: These studies will characterize a novel, fundamental mechanism of neutrophil activation, a key, early step of an inflammatory reaction. Knowledge from this work may provide an effective strategy to limit inflammation-associated damage not only in lung, but in all tissues.